1. Field of the Invention
The present invention generally relates to an information recording medium, and more particularly, to an information recording medium that stores an efficient sequence of multi-level random numbers as reference data, a method of generating the efficient sequence of multi-level random numbers, and a computer program for generating the efficient sequence of multi-level random numbers.
2. Description of the Related Art
Optical disk recording apparatuses that can store multi-level data in optical disks are developed. Reference data are sometimes stored in such an optical disk so that the optical disk recording apparatus can calibrate the write strategy and accurately reproduce multi-level data stored in the optical disk.
For example, in the case where each random number is 8-level number and three successive random numbers are considered as a sub-sequence, there are 512 (=83) different patterns. At least 514 (=83+3−1) random numbers are required to exhaust all of 512 patterns.
There are many methods to generate a sequence of multi-level random numbers. International Laid-open Patent Application WO01/57857 A1, for example, discloses a method of generating such reference data using a shift register circuit that generates 9-bit maximum-length sequences to calibrate the write strategy of an optical disk recording apparatus. This method can generate a sequence of 8-level random numbers by converting each 9-bit data randomly generated by the shift register circuit into three 3-bit data random numbers. The sequence thus generated can exhaust all of 512 patterns, but it is redundant as it will be described later.
On the other hand, the reference data are also used to reproduce actual multi-level data recorded on an optical disk using pattern recognition method. In this case, the optical disk recording apparatus reproduces the reference data and prepares a table of reproduced signal values. When actual multi-level data stored on the optical disk are reproduced, a signal value reproduced from the actual multi-level data is compared with that of the reference data using the table. The optical disk recording apparatus determines and outputs the pattern of the table of which the reproduced signal value best matches that of the reproduced actual data.
However, in the case of the former conventional art, the sequence includes 512×3=1,536 random numbers. 512 subsequences (each consisting of three consecutive random numbers) can exhaust all of 512 random patterns (of subsequences). However, if three consecutive random numbers starting with the second or third random number of each subsequence are taken into consideration too, the patterns of the three consecutive random numbers appear more than once. Therefore, the sequence of 1,536 random numbers is redundant. Additionally, since the technique uses the shift register circuit for generating maximum-length sequences, the number of multi-levels is limited to the powers of 2 such as 4, 8, and 16.
In the case of the latter conventional art, the redundancy in the reference data also matters. Additionally, because the reference data are not random enough, the reproduced signal may include a particular frequency element that affects the performance of servo mechanism of the optical disk recording apparatus.